JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 103, NO. E9, PAGES 20,149-20,158, AUGUST 30, 1998
Galileo ultraviolet spectrometer observations of Jupiter's
auroral spectrum from 1600-3200 A
o
WayneR. Pryor,
1 Joseph
M. Ajello,2 W. KentTobiska,
3 DonaldE. Shemansky,
4
GeoffreyK. James,
2 CharlesW. Hord,1 SmartK. Stephens,
3 RobertA. West,2
A. Ian F. Stewart,
1 WilliamE. McClintock,
1 KarenE. Simmons,
1
AmandaR. Hendrix,1andDeborahA. Miller1
Abstract. In 1996 and 1997 the Galileo UltravioletSpectrometer
(UVS) obtainedthe first meas-
urements
ofJupiter's
nightside
midultraviolet
(MUV)polarauroral
spectrum
from1620to3231/•
at13/• resolution.
ThereduceA
polarspectra,
afterremoval
ofoff-axis
scattered
radiation
fromthe
sunlitdaysideof Jupiter,containa spectrumthatmatcheslaboratoryspectraof theH2 continuum
in the a-b dissociative
emissiontransition.This is thefirst directidentificationof the H2 a-b tran-
sitionin astronomy.
Thea-bemission
isexcited
byelectron
impact
exch
.ange
reactions
withH2
thatpeakin crosssectionnear 15 eV. The emissionthresholdis at 1216 A, andthe continuum
peaks
inintensity
inthe2000-2500
3.range.
Jupiter's
observed
wavelength-integrated
MUVH2ab emissions
(1620-3231/•)
haveaphoton
flux-8 times
smaller
thansimultaneously
observed
wavelength-integrated
far-ultraviolet
(FUV)H2band
emissions
(1230-1650
3.).Because
theFUV
H2 emissionshavean emissioncrosssectionthatpeaksat higherenergiesnear50 eV, this
FUV/MUV brightness
ratio is diagnosticof the secondary
electronenergydistributionandis consistent with a "warm" distribution of electrons.
the HST Wide Field PlanetaryCamera2 (WFPC-2) FUV images
that have some sensitivity to the MUV [Clarke et al., 1996].
Jupiter'saurorahasbeen extensivelystudiedwith sounding The hydrocarbons
and aerosolsthat darkenthe FUV polar imrockets,spacecraft,and ground-based
telescopesin the X ray, agesand modify the auroralspectraare probablyproducedfrom
extremeultraviolet(EUV), FUV, andinfrared(seethereviewby methaneby the polar auroras.The aurorascontainmore energy
Ajello et al. 1998, [this issue]).Part of the energy deposited for methane dissociation than is available in sunlight for
by particle precipitation is reemittedin photons producedin
methaneLyman ot photolysisin the polar regions[Hord et al.,
transitions
in H, H2, CH4, C2H2, C2H6, H3+, andother gases. 1979; Pryor and Hord, 1991].
Emissions in the 500-1200 3, EUV and 1200-1700 3, FUV
TheMUV auroral
spectrum
from1700-3200/• wasprevispectralregions are dueto electron and ion impact excitation ously unexploredbecausereflectedsunlight from Jupiter's atof H2 and H. Recent Galileo ultraviolet spectrometer(UVS) mosphereoverwhelms the auroralemissions as seen from the
FUV Jupiterauroralspectraare remarkably similar to nearly vicinity of Earth. This is not true in the EUV and FUV because
simultaneouslow-resolution Jupiter FUV auroral spectraobthe solarflux is severalordersof magnitudeweaker than in the
tainedby J. Clarke with the Goddardhigh resolutionspectro- MUV andbecausethe H2 emissionsare brightest in the EUV
graphon the HubbleSpaceTelescope(HST) [Ajello et al., this and FUV. Galileo's uniqueability to observeJupiter'sdarkside
issue).GalileoEUV and FUV auroralspectraare differentfrom hasnow producedthe first MUV auroralspectra.This paperdean opticallythin laboratoryspectrumof electronsbombarding scribestwo typesof MUV darksidespectra.
H 2 becauseof self-absorption
effectsin H2 gas, absorptionin
1. Darksidespectraof Jupiterobtainednear90ø phaseangle
CH4 (methane) and C2H2 (acetylene), and absorption from that contain off-axis scatteredradiation from Jupiter'sdayside,
but couldbe obtainedwith a long integration period to obtain
larger hydrocarbonsand from aerosols[Ajello et al., this isreasonable signal-to-noise.
sue].Away from the auroralregions,the polesare quitedark in
2. Darkside spectra of Jupiter's southern aurora obtained
from within the shadowof Jupiter that contain less off-axis
scatteredradiation from the dayside, but have low signal-to1. Introduction
noise.
As we will show, the 90ø phase angle MUV polar auroral
spectra from the nightside have a distinct wavelengthJetPropulsion
Laboratory,
Pasadena,
Califomia.
dependentemission that is not dueto off-axis scatteredradiaFDC/Jet
Propulsion
Laboratory,
Pasadena,
Catifomia.
Department
of Aerospace
Engineering,
University
of Southem tion from the dayside.The MUV auroralspectrumis consistent
with laboratorymeasurements
of the H2 a-b continuumradiaCalifomia, Los Angeles.
tion obtainedby Jameset al. [this issue] in electron impact
experiments.Simultaneous
FUV/MUV spectraobtainedwithin
Copyright1998by the AmericanGeophysical
Union.
Jupiter'sshadowclearlyshowbrightFUV aurorabut provedinconclusivein the MUV due to shortintegrationtimesand a low
Papernumber98JE00787.
0148-0227/98/98JE-00787 $09.00
signal-to-noise ratio.
Laboratory
forAtmospheric
andSpace
Physics,
University
of Colo-
rado, Boulder.
20,149
20,150
PRYOR ET AL.' GALILEO UVS JUPITERAURORAL SPECTRUM1600-3200A
2. Instrumentation
Referencereflected .•,•)lurspectrum 1996-548
The Galileo UVS instrument[Hord et al., 1992] consists of
a Cassegraintelescopeand a Fastie-Ebertspectrometer.The
spectrometerusesa scanning grating drive to obtain spectra
with three different photomultiplier tubes. FUV spectra are
measuredwith a "G" tube with a CsI photocathodein second
6
order
from1133to 1921/• at 6.7 /•resolution.
MUVspectra
are measuredfrom 1620 to 3231 /• with 13.6 /• resolution in
first order using an "F" tube with a CsTe photocathode.Near-
ultraviolet
(NUV)spectra
from2820to 4320 /• aremeasured
with13.6/• resolution
in firstorderusingan"N"tubewitha
KCsSbphotocathode.
The G andN slits have the samefield of
view, measuring0.1 by 1.0ø on the sky.The F slit usedfor the
MUV spectradescribedhere is shorterin one dimension,measuring 0.1 by 0.4ø. The centersof all threeslits are boresighted,
that is, they all look in the samedirection. It requires4 1/3 s
to obtain an F-channelor G-channelspectrum,including a 1/3
s deadtime for large grating motions and a 4 s interval in
which smallergrating stepsoccur528 times 7.57 ms apart.
Actual data integrationlasts6 ms on each7.57 ms step.EachF
channel
stepis spectrally
displaced
by 3.1 /•, andsamples
lightfroma rangeof wavelengths
13.6 /• widefull-width
at
half maximum (FWHM). The F-channel recorded counts for
each step are always log-compressed
into 8-bit numbersby the
UVS electronics.We have adoptedthe in-flight stellar calibration of the F-channeldescribedby Hendrix [1996].
90 De9ree phase dark side auroral spectrum 1596-304
The data collection system can be configuredinto two
modes. The recorded mode was used for observations
obtained
with Galileo insideJupiter'sshadow.In recor• modeeach8bit element of data is storedseparatelyfor eventual transmission to Earth, allowing accuratereconstructionof the uncompressedtube counts at low count rates. Recordedmode is resource-intensive,requiring about 1 kbit of data to be downlinked for each secondof recording.
A secondmode, called real-time science (RTS), was developed in-flight when it becameapparentthat Galileo wouldbe
limitedto low dataratesby the failureof the highgain antenna
deployment.RTS useson-boardsummationof the instrumental
datato greatly reducethe amountof downlink telemetry. For
the F-channeldatathe generalapproachwasto sum the logcompresseddata at each wavelengthin the spacecraftcentral
computer,transmitit to the ground, and then divide by the
numberof integrations,
decompress
the data,andmultiply by
the numberof integrations.This algorithm is satisfactoryif
the sourceobservedin one integration is of constant brightness.
The U'VStelescopehas an external sunshadeand an extensive baffle systemfor rejection of off-axis light. The U'VSresponse is reducedfrom the on-axis responseby a factor of
-10,000 for a point source0.25ø off-axis in the directionorthogonalto the long slit, and by a factor of-30,000 at an angle of 0.5ø [Hard et al., 1992]. The off-axisrejectioncurve dictatesa strategyfor UVS MUV observations
of observingJupiter's auroraon the planetarydarksideas far from the terminator
as possible.In practice,this meansobserving the aurorasa
few slit widths(a few tenthsof a degree)from the terminatoron
Figure 1. Galileo UVS F- and G-channelfields of view illustratedfor two typical Jupiterobservationsnear 90ø phaseangle. The F-channel field of view is smaller and nested inside
the G-channelfield of view. (a) The observing geometryfor
the referencereflectedsolar spectrumobtainedon 1996 day
348 06:36-07:36 that wasusedin our modelingto represent
off-axis scatteredlight. (b) A darkside auroral observation
from 1996 day 304 07:38-08:07. These observations were
generallydoneas far on the darksideas possible to minimize
off-axis scatteredlight from Jupiter'sdayside.
wason-axis
forthereference
spectrum
contributes
-0.3 R//• of
off:axis scatteredradiation to a darkside spectrum obtained
thedarkside.For example,thedaysidereflectedlight spectrum 0.5ø away.
hasa brightness
of 9 kR/]•atthe2250]• peakof theH2 a-b
continuumfor the geometryin Figure la. (The spectrumob- 3. Observations
tainedat that geometrywasselectedas a "reference
spectrum"
3.1. Observations From Near 90ø Phase Angle
to representthe off-axis reflectedsunlight in our reduction
Table 1 lists the near 90ø phase angle MUV observations
process.)At an angle of 0.5ø the off-axis responseis reduced
by a factorof 30,000, so that the regionof the daysidethat examinedhere.Thesewerereal-timeobservationsinvolving
PRYORET AL.: GALILEOUVSJUPITERAURORALSPECTRUM
1600-3200
A
20,151
Table 1. ChannelF/G ScanObservations
FromNear90ø PhaseAn[le
SCE
Day
StartTime
Observation
Day
Record
Target
EndTime
(UTC)
SystemIT[
Latitude Longitude
(UTC)
1996
173
176
00 h07m14'
04 • 14m25'
173
176
00h36m33'
04• 16m27'
j0cd._glaura02
g01a_aurmap01
248
304
00h44m23'
01 • 13m49'
02 • 14m29'
03• 15m09'
04h 15m49'
05• 16m29'
06h 17m09'
07h38 m02'
08h08 m22'
08h38 m42'
248
304
00h50m27'
02h 13m28'
03• 14m08'
04• 14m48'
05• 15m28'
06h 16m08'
07h37m01'
08h07m21'
08h37"•41'
09h08m01'
g02a_aurmap01
g02c_c3aura01
g02c_c3aura01
g02c_c3aura01
g02c_c3aura01
g02c_c3aura01
g02c_c3aura01
g02c_c3aura02
g02c_c3aura02
g02c_c3aura02
09h38 m21'
gO2c_c3aura02
03h 17m28'
20h24m52'
c03a_aurmap01
c03c_e4aura02
20 • 25 m52'
20 h 55 m 12'
c03c e4aura02
20 • 56 m12'
21 h25 m32'
c03c_e4aura02
21 • 26 m32'
21 h 55 m52'
c03c e4aura02
21 • 56 m52'
22 • 27 m12'
23h27 m52'
22 h26 m 12'
23 • 26 m52'
00h27m32'
c03c_e4aura02
c03c_e4aura02
c03c_e4aura02
02 h 31 m54'
03 • 32m34 ,
03 h 31 m33'
04h 32 m 13'
c03c e4aura03
C03C e4aura03
04 • 33 m14'
05h32 m53'
c03c_e4aura03
05 h 33 m54'
06 h 33 m33'
c03c e4aura03
09 h09m02 '
308
347
347
348
03h06m21'
19• 25 m12'
308
347
348
06 • 36 m35'
-53.8
200.4
2
s_term
3
2
0
1a
2a
3
4
s_day
s_day
off planet
eq_night
eq_night
eq_night
eq_night
5b
s_night
-55.9
19.2
0
1
s_night
s_night
-57.0
45.8
-54.4
55.8
2
s_term
-54.4
64.7
3
s term
-55.3
76.8
2
s_day
0b
1b
2b
3b
4b
n_night
n_night
n_night
n_night
n_night
5
6
1
-55.3
53.0
-55.5
323.9
-1.7
264.0
-0.2
285.2
-1.4
308.7
-2.2
334.5
-52.6
82.7
1.4
120.9
65.2
203.1
62.6
186.5
62.5
193.5
62.2
199.2
n_night
61.2
204.7
n term
61.4
221.3
off planet
2b
3b
s_night
s_night
4
5e
2
s_night
s_day
s_day
-54.8
-55.4
91.1
-53.9
359.1
57.6
35O.5
61.7
37.5
-54.8
-54.8
70.2
84.4
103.0
07 h36m14'
c03c_e4aura03
351
11h33m19'
351
11h35m20'
e04a_aurmap01
049
11 • 43m07 '
049
12 h 43 m47'
e06a e6aura02
12• 45 m48'
13h45m27'
e06a_e6aura03
2
0
n_night
n_night
13 • 46 m28'
14 • 46 m07'
e06a e6aura03
1
n term
60.4
25.8
01h 18m04'
01h48m24'
e06a_aurmap01
e06a_aurmap01
0
1
s_night
s_night
s_night
s_night
s_mght
n_night
n_night
n_night
s_day
n_day
s_day
s_night
s_night
-56.6
100.8
-56.5
124.6
58.4
s_term
-64.8
333.7
n_day
n_day
59.2
205.7
58.6
157.9
61.0
179.1
1997
050
092
00h57m51'
01h 19m05'
14• 13m10'
15h 17m52'
050
092
16• 49m46'
17h21m14'
129
177
179
17h51m34'
18h21m54'
05•05m36'
07• 06m28'
07• 27m42'
08h38m07'
09h08m27'
129
177
179
14h42m29'
15h47m12'
g07a._g7aura02
g07a_fixtmd01
1b
1
17h20m13'
17h50m33'
g07a_aurmap01
g07a_aurrnap01
0
1
18h20m53'
18h42m07'
06h35m36'
07• 26m41'
07• 52m58'
09h07m26'
09• 37m46'
g07a_aurmap01
g07a_aurrnap01
g08a._g8aura04
c09a_aurmap03
c09a_aurmap03
c09a_aurmap04
2
3
1
0
1
1
c09a_aurmap04 2
n_day
-55.0
94.3
-55.6
74.7
195.9
56.2
181.8
-67.1
149.6
56.2
160.1
-64.4
64.4
-67.8
357.7
-61.7
3.9
Here n_day,northerndayside,s_term,southernterminator,eq_night,equatorialdarkside.All latitudesare planetocentric.
aIncludedin thedarksideequatorial
sununed
spectrum
of Figure7.
b Included
inthedarkside
polarsummed
spectrum
ofFigures
5, 6, and8.
c The selectedreferencereflectedsunlightspectrum,
fromthe southpolardayside.
altemate4 1/3 s MUV (F-channel)and4 1/3 s FUV (G-channel)
spectralscans,called F/G scans.Someobservationswere darkside observations
of the auroras that contain
off-axis
contami-
consumespacecraftfuel. Observationsat lower phase angles
providedtoo smalla view of Jupiter'sdarkcrescentto separate
the MUV aurorafrom the off-axis scatteredlight from the dayside. Lower phaseangle auroralobservationsfocusedon the G
nation from Jupiter's dayside.Someof the observationsconrainedJupiter'sdaysideandproducedreflectedsunlightspectra. channel(FUV), wherethe weakersunlightdoes not overwhelm
Theseobservations
wereall obtainedduringtheperiodon each the aurora.The latitudesand longitudeslisted are for the center
orbit when the spacecraftwas drifting past Jupiter at about 90ø of the slit. If the centerof the slit fell off the body, no latitude
phase angle. Observationsfrom higher phase angles usually and longitudeare given.In a few cases,two pointings occurred
requirespacecraftturnsto see Jupiter and are rare becausethey
within a singledata record,both of which are given in Table 1.
A datarecordis onereadout
(flush)of thespacecraft
RTSdata
C3 Dork %•de1996-314,•/05'48
buffercontaining
(inourMUV RTSdatasets)a singleF anda
singleG summedspectrum.
Thegeometry
of theauroral
ovalsisnotsymmetric
between
northandsouth;
thenorthern
ovalcontains
themostequatorwardsegment
of theauroralarcs,nearlongitude
180ø. Thusthe
maximumangulardistanceof the northernoval from the north
poleviewedfromGalileois largerthanthemaximum
angular
distance
of thesouthern
ovalfromthesouth
pole.Thisimplies
thatthemostfavorable
MUV observations
maycomefromthe
north,when180ø is nearthedarklimb,providing
thelargest
anglebetweenthe terminatorand the auroralemissionsin or-
der to minimizeoff-axislight.Figurelb illustrates
theobserv-
inggeometry
fora typical
darkside
observation
near90ø phase
angle.
3.2. Observations
From WithinJupiter'sShadow
On 1996day314 duringthe "Callisto3" (C3) orbit, the
Galileospacecraft
wasturned
to observe
Jupiter's
darkside
from
within Jupiter'sshadow.UVS-recorded
observationswereob-
tained
for66m 44 s during
thisturn,although
onlyhalfof this
wasreturned
dueto downlink
limitations.
TheUVSobserving
.3 Dc]rkSide 1996- 3147•-0•' .54
strategyselectedfor the southpolarobservationsinvolvedal-
b
ternating
4 1/3 s spectral
scans
of theF (MUV)andG (FUV)
tubes,
of which12m 8 s werereturned.
Figure2 showstheobservinggeometryfor the southpolarobservations.
The F-
channel
slitobserved
darkside
emissions
from100-130
ø SystemIII longitude.
TheFUVandMUVtotalcounts
perspectrum
asa functionof timeareshownin Figure3. The FUV count
rates peak when the slit was near the central meridian on the
darkside,
dueto auroral
emissions.
TheMUVcountratespeak
at thebeginning
of theobservation,
whentheslit waspointed
at the planetarylimb. Figure4 showsthe summedFUV and
MUV spectra
for this period.TheFUV spectrum
showsthe
usual
H2 bandemission
pattern
seen
inUVSlabspectra
of electronimpact
onH2 [Hordetal., 1992]andUVSJupiter
spectra
[Ajelloetal.,thisissue].
TheMUV spectrum
agrees
wellwitha
leastsquares
fit spectrum
formed
froma solarspectrum
[Van
Hoosieret al., 1988]multiplied
by the instrument
calibration.
Weconclude
that the MUV spectrum
seenhereis dueto for-
ward-scattered
sunlight.A similarfit is obtained
if a dayside
Jupiter
spectrum
fromUVSisusedinstead
of a solarspectrum.
Because
theG slitislongerthantheF slit, it is possible
that
the observed
FUV emissionsoriginatedoutsidethe field of
viewof theF slit.This low signal-to-noise
datasetis consis-
tentwiththeabsence
of anMUV aurora
andshows
thatany
MUV aurorais considerably
weakerthantheFUV aurora.
Better
MUV auroral
observations
wereobtained
at90øphase
thatallowusto quantify
theFUV/MUVbrightness
ratio.North
polarobservations
during
theC3 turnobtained
by alternating
theN (NUV) andG channels
didnotdetectauroralemissionsin
eitherchannel,confirmingthat thenorthernauroraloval was
notin thefieldof viewdueto a latetrajectory
update.
AdditionalNUV darkside
observations
from withinJupiter's
shadow were obtained on orbit C10.
Figure2. Viewinggeometryfor the F- andG-channelfields-
of-viewduringtheC3turnsouthpolarobservations
of Jupiter
on 1996 day 314 05:48-06:01. Galileo was in Jupiter's
shadow.
TheIo torusauroraloval andthemorepolarreference
auroraloval (thoughtto be the sourceof the emissions)are
shown(J. Clarke,personalcommunication,
1996). The geometryis shownat (a) the start(05:48), (b) the midpoint
(05:54), and(c) nearthe end(06:00)of the observation.
C.5[•(]rkS,de 1996 ,314•/•-•00
PRYOR ET AL.: GALILEO UVS JUPITERAURORAL SPECTRUM 1600-3200A
C3 Turn
G-Channel
200
Counts
+
150
• •
••
+
• •+ +
+ •
+•+ •
20,153
H2(XlZg+)
+ e ->H2(a3Zg+)
+e
(1)
H2(a3Zg+)
_>H2(b3Zu+
) +hv
(2)
+
100
+t ,•
H2(b
3Zg+)->
2H(ls)
+
50
0
52
50
48
54
56
58
60
62
Minutes after 1996 Day 314 5:00
C3 Turn
F-Channel
Counts
4OO
350
•
•
300
•
Two-photon continuumemission [Osterbrock,1974] from
metastableH(2s)atoms (lifetime against decay=0.12 s) producedby dissociativeexcitation of H2 is not consideredhere
becauseof rapid quenchingby atomic and molecularhydrogen
ions, by electrons, and by neutralsin the auroral deposition
region.In this two-photo,n
system,the probabilityof a 2s H
•+
•
•,+
•
+
atom becominga ls H atom by emitting a photon is symmet-
+
ric around
theemission
peakat 2431 /•. Bound-free
andfree-
250
free emission continua in H are also present in hot H,
2OO
48
(3)
50
52
54
56
58
60
62
Minutes after 1996 Day 314 5:00
Figure 3. Total countsin each 4 1/3 s spectrumobtainedin
the C3 turn south polar observations indicated for (top) the
FUV G channel and (bottom) the MUV F channel. Instrument
backgroundhas not been subtracted.
3.3. Laboratory Spectra
Since Jupiter's atmosphereis mostly H2 gas, we compare
the JupiterMUV auroralspectrumto the MUV spectrumemitted
by electronsbombardingH2 gas. Ajello et al. [1988, 1989]
constructedan electron gun with magnetic collimation. The
electronbeam is crossedwith a low-pressuregas beam, exciting variousmolecularand atomicemissions.James et al. [this
issue]usedthis instrumentwith H 2 as the target gas and obtained calibrated emission spectrain the wavelength range
[Osterbrock,1974] but are probablyrelativelyweak emissions
on Jupiterbecauseof the low H/I-I2 ratio expectedat auroralaltitudesand temperatures.
4. Comparisonof 90ø PhaseData and Model
We now describethe process of separating the nightside
auroralspectrumfrom the somewhatbrighter off-axis scattered
radiationfrom Jupiter'sdayside.Inspectionof the data set suggeststhat darksidespectraare generally similar to the dayside
reflectedlight spectra, but the darkest darksidepolar spectra
have a somewhatdifferentspectralappearance,suggestingthe
presenceof an auroralcomponent.To improve the signal-tonoise ratio, we summeddarksidepolar observations that had
relatively small radiation backgroundand off-axis scatteredradiation signatures.Figure 5 shows the stun of the north and
southpolar darksideMUV spectraobtainednear 90ø phase to
date, with the selection criterion that the count rate in channel
420at2910/!,,nearthepeakof theJupiter
dayside
spectrum,
was below 50 counts/s. Nine records of data met this criteria
1750-5300/• usingelectron
energies
of 14, 19, and100eV.
Jameset al. identified the emissions from 1600 to 3200 /• as
the H2 a-b continuumbandsystemthat radiatesat wavelengths
longerthan1216/• [AjelloandShemansky,
1993].TheH2
C5 Turn G-Channel Spectrum
40
triplet
a 3Zg+
state
isexcited
fromsinglet
ground
state
X •Zg+
by the only efficient mechanismfor populating the triplet
structure,electron exchange collisions with low-energy free
electrons.
Thea 3Zg+
statemaybeexcited
directly
fromthe
-', 20
10
groundstate,or by excitationof higher triplet states(also by
electron
exchange)
thatsubsequently
cascade
to thea 3Zg+
state.Thethreshold
energy
forexcitation
of thea 3Zg+
is
1200
5OO
1800
in (1).Theexcited
a 3y•g+
stateradiates
in anelectric
dipole
400
C3 Turn F-Channel Spectrum
600
transition
to therepulsive
b 3Zu+state,producing
H2 a-bcontinuum
emission
longward
of 1216/•andtwofastH(ls) atoms
200
each with a few eV of kinetic energy. The combined (direct +
lOO
(7 eV FWHM). The emission cross section has a maximum
1600
Wavelength(Angstrams)
11.82 eV. Below 20 eV electron impact energy, electron exchangeis the most likely processfor excitation of the a state
cascade)
emission
cross
section
to theb 3Zu+is rathernarrow
1400
sag
1800
2000
2200
2400
2600
2800
,5000
,5200
Wavelength(Angstrams)
valueof 5.0 x 10-17cm2 (1.73+_0.85
x 10-17cm2 dueto direct
excitation
and3.3x10'•7 cm2 dueto cascade)
at an electronim- Figure 4.
pactenergyof 15.5 eV [Ajello andShemansky,1993], in the
range of values characteristicof secondaryauroral electrons
[Opal et al., 1971]. At higher energiesabove 50 eV the cross
sectionfallsas 1/E3. Primaryauroralparticles
havetypicalenergiesof tensor hundredsof keV. At theseenergies,the primaries have negligible cross section for excitation from the X
Summedspectrafor the C3 turn southpolar observationsshown(top) for the FUV G channeland (bottom)for
the MUV F channel.The G-channelspectrum,shownas counts
perwavelength
step,shows
H Lymanat(1216/!,)andH2 band
emissions.The F-channelspectrumis very similar to a solar
spectrumthat has been multiplied by the instrumentin-flight
calibration [Hendrix, 1996]. A substantial instrumental back-
groundis alsopresent.To obtainreasonableF-channelsignal-
1Zg+
toa 3Zg+
state.
Thedirect
excitation
andemission
processto-noise,countsfrom 10 neighboringwavelengthstepshave
canbe summarizedas follows [Ajello and Shemansky,1993]:
been co-added.
20,154
PRYORET AL.: GALILEOUVS JUPITERAURORALSPECTRUM1600-3200
A
Examinationof a summeddarksideequatorialMUV spectrum
(Figure 7)reveals no obvious pattern other than the off-axis
daysidespectrum.The samereferencereflectedsolar spectrum
55OO
_.. 19 eV e- on H2 and Jupiter Day Side
.-,
5OOO
fitsthisspectrttm
with a reduced
Z2 of 1.17. In thiscase,add-
__
14
eV
eon
H
2and
Jupiter
Day
Side
•. I'
,
ing the 14 eV or 19 eV componentsto the regressiondidnot
ß
500
..
Jupiter
Day
Side •/ [- -1
__Polor
Night
Side
-J "'
!1
':..
•"i
u
4OOO
3500
2000
2500
3000
change
thequalityof fit Z2 significantly:
'the newfits also
haveZ2 of 1.17.Weinterpret
thelackof a 14 eV or 19 eV
componentin the equatorialspectrumas furtherevidencethat
the high-latitudedeviationin the MUV spectrumfrom the offaxis daysidespectrumis a phenomenoncausedby the polar
auroras.We also experimentedwith usingother daysidespectra
asthereferencesolarspectrum.While thereweresubtledifferencesbetweendaysidespectra,the choice of spectrumdid not
affect the conclusionthat an MUV aurorais present at high
latitudes.
The significanceof the MUV polar auroraldetectioncan be
discussed
in severalways. The presenceof a large off-axis reFigure 5.
SummedF-channel (MUV) darksidenorth and
flected solar contribution in the spectrumraises the issue of
southpolar spectrumfrom 90ø phase.Three multiple linear regressionfits are also shown: (1)a fair fit using just a back- potential systematicerrors.The presenceof a polar H2 a-b
in the dataand its absenceat the equatoris plausiground-subtracted
Jupiter polar daysidespectrttmto represent component
ble in terms of our FUV auroralexperience.Anotherpossible
the off-axis light contribution; (2)a better fit using the offaxis contributionanda 14 eV e- on H2 laboratoryspectrum; concernis variations in the reflected light spectrum.We adand (3) a similar fit usingthe off-axis contributionand a 19 eV dressedthese issuesby varying the referencesolar spectrum
e- on H2 laboratoryspectrum.To obtain reasonableF-channel andcontinuedto find the H2a-b componentin the data.An adsignal-to-noise, counts from 10 neighboring wavelength ditional questionis the behavior of the insmnnentoff-axis.
stepshave beenco-added.
We recently studiedthe UVS engineering model spectral responseas a function of angle by placing Hg andD2 lamps at
the focusof a collimator to producea plane wave at the UVS
instrument.Spectraobtainedwith the UVS aligned with this
beam (on-axis) and at an off-axis angle of 0.6ø wereremarkaand are so marked in Table 1. This summedspectrumhas a
spectralwavelengthdependencewith relativelymore emission bly similar in relative spectralintensity.Featuresdid not shift
at wavelengths
below2800]k thandoesthedayside
spectrum. in wavelength.The off-axis spectrumis much dimmerthan the
We examined multiple linear regression [Berington, 1969]
models of this spectruminvolving various regressioncomponents that included a wavelength-independentbackground,a
MUV Auroro
referencedayside spectrum, and laboratory electron impact
400
'- .......
...•00I-- ' • I
_ OorkSide- Scoled
OoyS,de
spectra.First, we fit the summedspectrumwith only a referencedaysidespectrum,where the solar reflectedlight is domiõ lOO
nant and has high signal-to-noise. This regression in the
o
0
softwarepackageInteractive Data Language(IDL) producesa
Wovelength(Angstroms)
o
"reduced
andweighted"
Z2 of 1.13,butis a fairlypoorfit, with
-100
-200
thismodel larger than the data at the shortest wavelengthsand
2000
2500
smaller
thanthedatain thewavelengths
near2300J• where
H2
emissionis expectedto be visible. If the fitting function is a
good approximation to the parent function, then the value of
M UV Au ro ro
400
throughout
thespectrum
witha reduced
Z2 of 1.02. A regres-
õ
o
-
' -
'
•
T.-.-•
_ pork
Side
- Scoled
Ooy
Side
lOO
o
0
-100
-200
2000
sion with both the referencedaysidespectrumand a 19 eV
laboratory spectrum(multiplied by the UVS responsecurve)
produces
a somewhat
worsefit, witha reduced
Z2 of 1.06. The
....
300
thereduced
Z2should
be approximately
1. Next,wetrieda regressioninvolving both the referencedaysidespectrumand a
calibrated 14 eV laboratory spectrum(multiplied by the UVS
responsecurve). This model producesa visually appealing fit
3000
Wovelength(Angstroms)
2500
3000
Wovelength
(Angstroms)
14 eV and 19 eV laboratoryspectraare sufficientlysimilar that
the regressionprogram by itself could not meaningfullyassess
their relative contributions.Figure 6 shows the summeddarkside spectrumafter subtraction of (1) the solar component
when the 14 eV specmnnwas the other componentin the regressionand (2) the solar componentwhen the 19 eV spectrum
was the other component. The nonsolar component of the
darksidespectrumstronglyresemblesthe 14 and 19 eV laboratory spectra,with a somewhatbettermatch for the 14 eV spec-
Figure 6. Jupiterdarksidepolar MUV spectrumfrom Figure5
after subtractionof the solar contribution, leaving just the
auroralcomponent.In the upperpanel,the solarpart of regression (2) just describedhas been subtractedandis comparedto
the 14 eV e-on H2 laboratory spectrum[Jameset al., this issue]andin the lowerpanel,the solarpart of regression(3) just
describedhasbeen subtractedand is comparedto the 19 eV eon H2 laboratoryspectrum[Jameset al., this issue].The fit in
the upperpanel is slightly superior. Again, countsfrom 10
neighboring wavelength steps have been co-added.The
statistical uncertainty in the 10 grating step wide bin is
trum.
shown.
PRYORET AL.:GALILEOUVSJUPITER
AURORALSPECTRUM
1600-3200
A
6000
20,155
catesthatvaluesof F• > 10.8have a less than 0.001 chanceof
occurringby chancewhen the numberof degreesof freedomis
large.We concludethat the additionalterm, the 14 eV H2 a-b
_.. 19 eV e- on H2 ond Jupiter Day Side
laboratory spectrum,is significant.
5500
__ 14 eV e- on H2 and Jupiter Doy Side
..
5000
Jupiter Doy Side
5. MUV and FUV Brightnesses
__
Equotorlol
Night
Side
'--'-•"1•
••"'"'L""""•R•
•
The near 90ø phaseangle data wereusedto find the relative
brighinessof the FUV and MUV auroras.Figure 8 shows the
summedFUV polarspectrumobtainedsimultaneouslywith the
...........
4500
summed
MUV polarspectrmn
of Figure5, ThisFUV polarspectrumis similar to a 100 eV e-on H2 FUV spectrumobtained
with the UVS in the laboratory [Ajello et al. 1988]. Below
4000
about1400/•, thespectra
differdueto methane
absorption
2000
2500
5000
Wovelength(Angstroms)
Figure 7. SummedF-channel (MUV) darksideequatorial
spectrumfrom 90ø phase.Three multiple linear regressionfits
are alsoshown:(1) a fair fit using just a Jupiterdaysidespectrum to representthe off-axis light contribution,(2) an identical fit usingthe off-axis contributionand a 14 eV e- on H2
laboratoryspectrum,(3) an identicalfit usingthe off-axis contributionanda 19 eV e- on H2 laboratoryspectrum.To obtain
reasonableF-channelsignal-to-noise,countsfrom 40 neighboring wavelengthstepshave been co-added.
from gas located above the auroral emissionsand due to differencesin the electronenergy distribution[Ajello et al., this issue]. DeliberatelyexcludingLyman or,we find the total number
of G-channel
(FUV)countsbetween1230 and1650 ]• (after
backgroundsubtraction) is 40,839. The total number of F-
channel (MUV) counts (after subtractinga wavelengthindependentbackgroundand the off-axis daysidescatteredradiation contribution) is 5836. To estimate the relative emission rates requiressome assumptions,becausethe G-channel
slit is longerthan the F-channelslit. If the two fields of view
arefilled, the wavelength-integrated
signal in the G-channel
(hererestricted
to 1230-1650•) is 5.8 kiloRayleighs
(kR),
andin theF-channel
(1616.5-3227.9
]•)is 1.8kR. In reality,
on-axisspectrum[Hord et al., 1992].The lab work and the polar/equatorialcomparison
provideconfidencein our dataset.
The statisticalsignificanceof the detectioncan be discussed
in two ways. First, how many standarddeviations is the detection? The summedpolar F-channelspectrumcontains230,772
counts, including solar reflectedsunlight, instrumentalbackground,and any signal. There are 5836 excess counts in the
summedpolar spectrumattributedto the H 2 a-b continuum.
This leads to an estimated12-13detection. A secondapproach
to assessingthe significance of the detection involves the Ftest [Bevington, 1969]. This techniqueaddressesthe following question:given a dataset that can be fit by two components (here, the instrumentalbackgroundand the reflectedsolar spectrum)or by three components(addingin the H2 a-b
continuum),how significant is the third component? Define
thegoodness-of-fit
criterion
Z2fora datasetwithN datapoints
the auroralarc is probablyquitenarrow(-200 km) in width and
fills a small fraction of each slit. The actual G-channel/F-
channelbrighinessratio in the arcis better approximatedby
(5.8/1.8)(1.0ø/0.4ø)=8.1, where 1.0ø and 0.4ø are estimates for
the effectiveslit lengthsof the G and F channels,respectively.
This correctionis invalidif part of the emissionoccursin portions of the G-channelslit that extendbeyondthe region of
overlap with the F-channel slit. In the calibrated 14 eV e-on
H2 laboratory spectrum the brighiness ratio between G-
channel(1230-1700/•)and F-channel
(1616.5-3227.9
wavelengths is 1.3, at 19 eV the ratio is 2.25, and at 100 eV
•2oo
•ooo
(N=528 here), Yi, andmodelpoints, ymi, andstatistical stan-
dard
deviations
0.•
800
Z2=i=l
• 0'/2
1...•_[y
i_Ymi
]2
!i:
i'
..Lob
100
eV
e-on
H
2
•• 600
i..
•
400
and the degreesof freedomv=N-n-1, for a fit to a function with
n coefficientsplus one constant term. Then the F-test for the
validity of an additionalterm is defined as
200
-
1200
FZ X2(n)
I(N- n-1)
For the comparisonof the two-term fit to the three-termfit
(with a 14 eV • a-b spectrum)for the summedMUV polar
1300
1400
1500
1600
1700
Wovelength(Angstroms)
Z2(n-1)-Z2(n)
Figure 8.
The JupiterdarksideFUV summed
auroralspec-
trum obtained simultaneouslywith the dimmer MUV auroral
spectrum
in Figure6, compared
to the UVS laboratoryspectrumof 100 eV e- on H2. Short-wavelength
differencesareattributedto CH4 absorption
in Jupiter's
atmosphere
andto electron energy effects.Note the absenceof reflected solar contri-
spectrum,
wefindF• 59.9.Berington[1969,TableC-5]indi- butionsto the FUV spectrum.
20,156
PRYORET AL.:GALILEOUVSJUPITER
AURORALSPECTRUM
1600-3200
A
the ratio is 9.9. The ratio at 100 eV is lower than that expected
from the (a-b) modelof Ajello and Shemansky[1993]. The un-
bution differences,and the effect of hydrocarbonabsorption
on the FUV auroraloutput. We also calculatethe photon flux
expectedlystrong H2 (a-b)spectrumat 100 eV can be explained by (1) a large numberof low energy secondaryelectronstrappedin the collimatingmagneticfield that efficiently
excite the continuumor/and (2) a very strong cascadechannel
from the many upper-lying triplet states that contains a large
singlet-triplet mixing. The arc-width estimate of 200 km
comes from imaging of the spatially resolved arc by the
Galileo solid state imager (SSI)in visible and near-infrared
wavelengths(A. Ingersoll et al., Imaging Jupiter's auroraat
visible wavelengths,submittedto Journalof GeophysicalRe-
ratioFUV Lyman(1500-1650
]i)/•UV (1616.5-3227.9
]i) because
theFUVabove
1500/•haslesshydrocarbon
absorption
search,1998).TheF-channel
regionfrom1616.5to 3227.9]i
ratioFUVLyman(1500-1650]i)/MUV(1616.5-3227.9
]i)is
comprises-75% of the photonsemittedin the whole H2 (a-b)
3.2, very close to the dataratio of 3.9. Table 2 summarizes
[Ajello et al., this issue]. In this case, we find the theoretical
ratio of 1.1 and the data ratio of 3.9.
The detected MUV
emis-
sion is less than expected. A 27-eV Maxwellian electron distribution improves the FUV spectral shape for some auroras
[Ajello et al., this issue].Convolving this proposedhotter 27eV distribution with the crosssectionsgives an FUV (1230-
1650]i)/MUV(1616.5-3227.9
]i) photonfluxratioof 7.5,
very closeto the measuredratio of 8.1. The 27 eV photon flux
system
(1216-oo
]i) if thespectrum
is dueto 14 eV electrons theseratios for the Opal et al. [1971] distribution, and27 eV,
striking H2.
50 eV, and 100 eV Maxwellian distributions, showing the
bandratios have a large sensitivity to the electron temperature. The Opal distxibutionis referredto as "cold", and the 100
eV monoenergeticor Maxwellian is "hot". We concludethat
the FUV/MUV ratios are consistent with the presenceof a
"warm" auroral electron distribution, with a temperaturenear
[this issue]representation
f(E), the flux of secondary
electxons 27 eV. Becausethe slit lengthsare different and the crosssec-
The measuredratio of FUV/MUV brightness can be comparedto theoretical expectations.The secondaryelectron energy distributionmeasuredin the laboratoryincreasesat lower
energiesand can be representedas a sum of 4 Maxwellians
[Opal et al., 1971; Ajello et al., this issue].In Ajello et al.'s
tions still have uncertainties near a factor of 2, this result is
of energyE in unitselectrons
eV4 s'l, canbe represented
as
not yet decisive, but is suggestiveof the presenceof warm
4
electrons.
fiE)= Y.Cn *(E/En)*exp(-E/En)
6. Discussion
with coefficientsC, for Maxwelliansof temperatureT, and energy E,=kT, wherek is Boltzmann'sconstant.The coefficients
To the bestof our knowledge,Galileo UVS has providedthe
first special observationof the H2 a-b continuumemissionin
any astrophysicalobject. The H2 a-b band systemmay be an
important processfor (1) heating outer planet atmospheres,
areC•=2.21,C2=0.406,
C3=2.84x10
-2,andC4=9.84x10
-4.The
correspondingtemperaturesin K are T•=3.25xl04,
T2=l.13x105,
T3=4.58x105,
andT4=2.09x106.
Thisequationmolecular clouds in
fits dataon secondaryelectronsin H• with energiesof 4-1000
eV and shouldnot be usedfor lower energies. The secondary
electron spectrumshapeis generally independentof primary
particle energy,provided one considersejectedelectronswith
energiesconsiderablylessthan half the primary energy [Opal
et al., 1971]. Computing the expectedbrightness ratio is a
matter of convolving the relevant absoluteemissioncrosssections with the expected secondaryelectron spectrum. Direct
electronexcitationof the B andC statesof H2 responsiblefor
the FUV B-X Lyman and C-X Werner band emission systems
has a peak crosssectionfor electronswith energiesnear 50 eV
[Liu et al., 1998]. The a state, responsible for the H 2 a-b continuum, with its narrowly peaked electron excitationcross section of 15.5 eV [Ajello and Shernansky, 1993], samples
somewhatlower energy electrons. The convolution procedure
involving model a-b crosssectionsleadsto an expectedFUV
(1230-1650
]i)/•UV (1616.5-3227.9
]i)photonfluxratioof
2.5. The measuredratio of 8.1 is close enoughto supportthe
claimeddetection. Discrepanciesmay reflect the differentslit
lengths,crosssectionuncertainties,secondaryelectrondistriTable
2.
Model
emission
the interstellar medium, and stellar atmos-
pheresof cool stars and (2) measuringthe secondaryelectron
energies.Fast atomic hydrogenH(ls) atomsmade in the a-b
transitionwith characteristicenergiesof 3 eV (23,000 K) will
heat Jupiter'satmospherethroughcollisions. Diagnosingthe
secondaryelectronspectrumis also of importancebecausethe
secondariesare responsiblefor most of the emissions[Rego et
al., 1994]. It is not too surprisingthat the MUV auroralspectrum resemblesthat of H2 excitation by elect•ons with 14 eV
impact energy. H2 is the most abundantgas in the Jupiteratmosphere.The manifoldof triplet statesof H2 has large excitation cross sections for the a, b, c, d, e states in the threshold
regionnear 15 eV [Ajello and Shemansky, 1993; Khakoo and
Trajmar, 1986]. The triplet states(c, d, e.... ) with higher excitation energy than the a staterelax by dipole allowed radiative
transitionsto the a state, followed by an allowed transition to
the b state. MUV spectraldata are sensitive to electron energiesnear 15.5 eV, the peak in the a-b emission crosssection.
Simultaneouslymodelingthe FUV and MUV brightnesseshas
providedan important test of the secondaryelectron distribution, with the surprisingresult that a warm elect•on distribu-
ratios from different
electron dis-
tributionscomparedto UVS dataratio.
Photon Flux Ratio
FUV(1230-1650
30/ o FUV(1500-1650/[)/
MUV (1616.5-3227.9A)MUV (1616.5-3227.9/•)
GalileoUVS polardata
2C•aldistribution
eV distribution
50 eV distribution
100 eV distribution
8.1
2.8
7.5
16.9
43.7
3.9
1.3
3.4
7.5
19.5
PRYORET AL.: GALILEOUVSJUPITERAURORALSPECTRUM
1600-3200
A
tion with energiesnear 27 eV is consistentwith the brightness
ratio.
The GalileoUVS JupiterMUV spectraagreewith the laboratory dataon electronimpacton H2 quite well. However, in the
FUV, substantialhydrocarbonabsorptionsmodify the auroral
spectrumsubstantially [Ajello et al., this issue]. Reflection
from below may also be important. Yung et al. [1982] estimatedthata -20% enhancementof the FUV Lyman bandsoccursdue to reflectionof downwardflux by Rayleigh scattering
of H2. We now considerpossiblecorrectionsfor MUV absorptions and reflections.The reflectedsunlight spectrumof Jupiter in the MUV was measuredby the International Ultraviolet
Explorer (IUE) [Wageneret al., 1985] and by Galileo UVS
[Hord et al., 1995]. At Galileo UVS resolution, the reflected
sunlightspectrumis modifiedby the presenceof aerosols,but
no characteristicabsorptionscan be resolved.At IUE resolution, NH3 bandsare seen [Wageneret al., 1985]. However,
ammoniaabsorptionoccursbelow the NH3 cloudlayer at the
several hundred millibar level; this is well below the auroral
20,157
Acknowledgments.
The UVS team wouldlike to thank
the Galileo flight team for making possib16these uniqueobservations.This researchwas supportedby the NASA/JPL
Galileo Project. We acknowledgehelpful discussionswith
Randy Gladstone,Andy Ingersoll, and JohnClarke.
References
Ajello, J. M., and D. E. Shemansky,Electronexcitationof the
H2(a3•g+_>b 3Eu+
) continuum
in thevacuum
ultraviolet,
Astrophys.J., 407, 820-825, 1993.
Ajello, J. M., et al., Simple ultravioletcalibrationsourcewith
referencespectraand its use with the Galileo orbiter ultraviolet spectrometer,Appl. Opt., 27, 890-914, 1988.
Ajello, J. M., G. K. James,B. O. Franklin, and D. E. Shemansky, Medium-resolutionstudiesof extremeultraviolet emissionfrom N2 by electronimpact: Vibrational perturbations
andcrosssections
of theC4' l•u+ andb' l•u+ states,Phys.
Rev. A, 40, 3524-3556,
1989.
emissionsat pressuresnear the microbar level. Hydrocarbon
and aerosol absorptionsremain possible in the auroral spec- Ajello, J. M., et al., Galileo orbiter ultraviolet observationsof
Jupiter aurora,J. Geophys.Res., this issue.
trum, but basedon the Galileo reflectedsunlight spectrumwe
expect to see nearly grey continuumabsorption. Wagener et Belton, M. J. S., et al., The Galileo Solid State Imaging Experiment, Space Sci. Rev., 60, 413-455, 1992.
al. [1985] modeledthe IUE equatorialMUV continuumabsorptionfrom2400to 3200/• withtwohazelayers:a 1.5 optical Bevington, P. R., Data ReductionandError Analysis for the
Physical Sciences,McGraw-Hill, New York, 1969.
depthhazehomogeneouslydistributedfrom 150 to 600 millibar with a constant single-scattering albedo of 0.42; and a Clarke, J. T., et al., Far-ultraviolet imaging of Jupiter's aurora
and the Io "footprint",Science,274,404-409, 1996.
high (50 millibar), thin (0.25 optical depth) haze of singleCook,
A. F. II, A. V. Jones, and D. E. Shemansky, Visible
scatteringalbedo0.97. Wagenerand Caldwell [1988] next exaurora in Jupiter's atmosphere?,J. Geophys. Res., 86,
aminedthe latitudedependence
of the continuumabsorptionin
8793-8796,
1981.
IUE spectraandfoundtheupperhazeoccurshigherin the polar
Hendrix,
A.
R.,
The Galileo Ultraviolet Spectrometer:
In-flight
atmosphere,somewherein the 0-50 millibar region. These
calibration
and
ultraviolet
albedos
of
the
moon,
Gaspra,
Ida,
hazelayersmaybe belowthe aurorabut will affectany auroral
lightreflected
frombelow.IUEspectra
from2000to 2600/1• and Europa,Ph.D. thesis,Univ. of Colo., Boulder, 1996.
were slightlyred at high latitudesbut slightlyblue at the equa- Hord, C. W., R. A. West, K. E. Simmons, D. L. Coffeen, M.
Sato, A. L. Lane, and J. T. Bergstralh,Photometricobservator. Thesereflectionspectraare sufficientlygrey that correcttions of Jupiterat 2400 angstroms,Science,206, 956-959,
ing for theeffectsof absorption
on the smallreflectedcompo1979.
nent doesnot significantlyalter the MUV spectrum.
UVS auroralspectraof Jupiterin the still-unexploredNUV Hord, C. W., et al., Galileo Ultraviolet SpectrometerExperi-
(3500-4300/1•)
on orbitsC3 andC10 weremadewithGalileo
ment, Space Sci. Rev., 253, 1992.
in Jupiter'sshadow.We are currentlystudyingthese data,
alongwithNUV auroraldataobtainedat 90øphaseon orbitE6.
Fulxtremeasurements
are planned in the Galileo EuropaMis-
Hord, C. W., et al., Direct observations of Comet Shoemaker-
sion. Initial work shows the off-axis scatteredradiation problem is more severein the NUV becauseof the large solar flux.
James,G. K., J. M. Ajello, andW. R. Pryor, The MUV-visible
spectrumof H2 excited by electron impact, J. Geophys.
Levy 9 fragment G impact by Galileo UVS, Geophys. Res.
Lett., 22, 1565-1568, 1995.
Res., this issue.
NUV spectraof electronsstriking• gas aredesirablebecause
they have structurefrom allowedsinglet state transitionsto Khakoo,M. A., and S. Trajmar, Electron-impactexcitation of
theB andC states,have a strongenergydependence
from optithea 3Zg+,
B 1I;u+,
c 3YI
u, andC 1yI
u states
of H2,Phys.
Rev., 34, 146-156, 1986.
cally forbiddenexcitationto the GK, H, andEF states,and may
be evenmorediagnosticof the secondaryelectrondistribution Liu, X., D. E. Shemansky, S. M. Ahmed, M. Ciocca, G. K.
James,and J. M. Ajello, Electron impact cross sectionsof
[Jameset al., this issue].The Voyager cameraimagedJupiter's
the Lyman and Werner band systemsof hydrogen, Astrodarksideauroraandfoundan intensity of -20 kR of emission,
phys. J., in press, 1998.
but the emission wavelengths were not determined[Cook et
al., 1981]. More recently,the Galileo SSI [Beltonet al., 1992] Opal, C. B., W. K. Peterson,and E. C. Beaty, Measurementsof
secondary-electron spectra produced by electron impact
has been imaging Jupiter'snightside aurorawith a charged
ionization of a number of simple gases,J. Chern.Phys., 55,
coupleddevice(CCD) anda variety of narrowbandinterference
4100-4106,
1971.
filters and finds emissionsin all filters, including the NUV,
the visible, and the near infrared (A. Ingersoll, private com- Osterbrock,D. E., Astrophysics of GaseousNebulae, W. H.
Freeman, New York, 1974.
munication,1997). A Jupiterpolar orbiter missioncould make
gooduseof a spectrometer
with EUV, FUV, MUV, NUV, and Pryor, W. R., and C. W. Hord, A study of photopolarimeter
systemUV absorptiondataon Jupiter, Saturn,Uranus, and
visible capability for simultaneouslyidentifying the darkside
Neptune- Implications for auroralhaze formation, Icarus,
emissionsin optically allowedand forbiddenexcitation processes.
91, 161-172, 1991.
20,158
PRYORET AL.: GALILEOUVS JUPITERAURORALSPECTRUM1600-3200
A
Rego, D., R. Prange, and J.-C. Gerard, Auroral Lyman-ot and
H2 bandsfrom the giant planets, 1, Excitation by proton
precipitation in the Jovian atmosphere,J. Geophys. Res.,
99, 17,075-17,094, 1994.
Van Hoosier, M. E., J. F. Bartoe, G. E. Brueckner, and D. K.
Prinz, Absolute solar irradiance 120 nm-400 nm, Astro-
phys. Lett., 27, 163-168, 1988.
Wagenet, R., and J. Caldwell, Strong north/south asymmetry
in the Jovian stratosphere,Icarus, 74, 141-152, 1988.
Wagenet, R., J. Caldwell, T. Owen, S.-J. Kim, T. Encrenaz,and
M. Combes, The Jovian stratospherein the ultraviolet,
Icarus, 63, 222-236, 1985.
Yung, Y. L., G. R. Gladstone,K. M. Chang, J. M. Ajello, and
S. K. Srivastava,H2 fluorescencespectrumfrom 1200 to
1700J[by electron
impact:
Laboratory
studyandapplication to Jovian aurora,Astrophys.J., 254, L65, 1982.
J.M. Ajello,G.K. James,and R.A. West, Jet Propulsion
Laboratory,
MS 183-601, 4800 Oak Grove Drive, Pasadena,CA 91109.
A.R. Hendrix, C.W. Hord, W.E. McClintock, D.A. Miller, W.R.
Pryor,K.E. Simmons,
and A.I.F. Stewart,Laboratoryfor Atmospheric
and Space Physics,1234 InnovationDrive, Universityof Colorado,
Boulder,CO 80303-0590.(e-marl:wayne.pryo•lasp.colorado.edu)
D.E. Shemansky,
Departmentof AerospaceEngineering,University
of SouthemCalifomia, Los Angeles,CA 90089.
S.K. Stephens
andW.K. Tobiska,FDC/JetPropulsion
Laboratory,
MS
264-580, 4800 Oak Grove Drive, Pasadena,CA 91109.
(ReceivedOctober1, 1998;revisedMarch4, 1998;
acceptedMarch 6, 1998.)